Scroll compressors have orbital scrolls which are driven in a generally circular orbit. These orbital scrolls include a plate with a flat surface that is perpendicular to a rotation axis and an involute wrap integral with the plate and extending out from the flat surface. A non-rotatable scroll including a plate with a flat surface that is parallel to the flat surface of the orbital scroll and an involute wrap integral with the plate and extending out from the flat surface cooperates with the orbital scroll to form at least a pair of fluid pockets. The fluid pockets are bound by adjacent surfaces of the wraps, line contacts between the wraps and contact between the axial tips of the wraps and the flat surface of the adjacent scroll. A seal is normally provided in a groove in the axial tip of each scroll wrap to seal between the wrap and the flat surface of the adjacent scroll. Axial tip seals are provided to accommodate thermal expansion of the scroll end plates and the scroll wraps.
The orbital scroll is driven to cause the contact lines between the wraps to move along the surface of the wraps toward the center of the scrolls. As the contact lines move, the fluid pockets move toward the center of the scrolls, the pockets become smaller and the fluid in the pockets is compressed. A fluid outlet aperture is provided in the center portion of one of the scrolls.
The compressed fluid in the scroll pockets exerts an axial force on the parallel flat surfaces of the scroll end plates. This force tends to separate the scrolls and cause leakage of compressed fluid between the axial tips of the scroll wraps and the flat surface of the adjacent scroll. The force of compressed fluid also tends to distort the scroll end plates with flat surfaces. The distortion results from the fact that the. radial outer edges of the scroll plates are at compressor inlet pressure and the center of the scrolls is at the higher compressor outlet pressure.
The scrolls in some compressors are subject to a continuous axial thrust load which is sufficient to hold the scrolls together when operating at maximum output pressure and minimum inlet pressure. The scrolls in these compressors have excessive axial thrust loads on the scrolls at every operating speed and outlet pressure but the designed maximums. This results in excessive and unnecessary wear on compressor parts including seals, scrolls and orbital scroll drives. It also results in excessive power requirements and reduced efficiency due to heat generation.
Scroll type compressors have been built, for stationary refrigeration systems, which employ fluid at compressor outlet pressure to apply an axial thrust load to the center portion of the backside of a scroll plate and subject the radially outer portion of the scroll plate back side to fluid at compressor inlet pressure. It is even known to apply axial thrust loads to a portion of a scroll between the center and the radially outer edge by bleeding fluid from fluid pockets at an intermediate pressure into a toroidal chamber on the back side of the scroll plate. Such a system limits axial thrust loads on the scrolls to a minimum at all operating speeds and conditions, reduces wear and power requirements to a minimum, reduces scroll distortion due to uneven loading and increases compressor life. Unfortunately past systems of this type are expensive to manufacture, difficult to assemble and too large for use on mobil machines.